It is well-known that at the end of the primary step of recovery of petroleum from a petroleum reservior, in which step the recovery occurs thanks to the natural energy stored in the petroleum reservoir, or with the use of mechanical energy, inside the reservoir still a large amount of petroleum remains. Therefore, in order to increase the amount of petroleum which can be extracted, techniques of secondary recovery are customarily used, which essentially consist in injecting into the reservoir a fluid, generally water or an aqueous polymeric solution, or a gas, such as carbon dioxide or steam, which conveys petroleum to the production well.
However, owing to the heterogeneity of the petroleum bearing rock, which is constituted by regions with different permeability, such fluids tend to preferentially flow through the higher-permeability regions. The low-permeability regions remain hence non-fluxed, or only partially fluxed, and this matter of fact prevents the extraction of the therein contained petroleum.
A proposal of solution to overcome such a problem consists in modifying the permeability of the reservior either completely or partially occluding the high-permeability regions, so that the flow of the fluid subsequently injected into the reservoir for the recovery of petroleum can be diverted towards the low-permeability regions.
For that purpose, one may resort to the injection into the reservoir of an aqueous solution of a gellable polymer, which can be gelled by means of a multivalent metal ion, to cause polymeric gels to be generated in situ.
In particular, the use is known in the art, of gellable compositions having delayed gelling time, which compositions generally consist of a crosslinkable polymer, such as, e.g., a polyacrylamide, a multivalent metal ion, and a ligand or sequestering agent for said multivalent metal ion. For example in European Patent Applns. Public. No. 0 390 279 and No. 0 390 282, the use is disclosed of compositions which can be gelled by means of trivalent chrome, which compositions contain, as the retardant agent, an organic ligand selected from the group consisting of the aliphatic or aromatic dicarboxy acids, or from the group consisting of the alpha-hydroxyacids or alpha-aminoacids.
In UK patent application 2,187,773, a gellable composition having a delayed gelling time is disclosed, which contains a water-soluble polymer, a crosslinking agent constituted by a complex of trivalent chrome with a carboxylate ion, particular acetate ion, and a retardant agent, generally constituted by acetic acid.
Unfortunately, such compositions known from the prior art allow delayed gelling times to be obtained, which are generally unsatisfactory for practical uses. In particular, the compositions known from the prior art are, in general, unsatisfactory when they are used to occlude large regions of a reservoir and/or regions far away from the injection well, in particular when reservoirs are processed which display high temperatures of their own, which accelerate the gelling rate, with the risk of a premature gel formation, and consequent occlusion of undesired regions.